Improve custom loader documentation

[robwalch]

This PR will...

Improve custom loader documentation

Why is this Pull Request needed?

Resolve recent loader issues:

• #7007
• #7012
• #7010
• #7022

Are there any points in the code the reviewer needs to double check?

Recent issues have been labelled "Custom loader": https://github.com/video-dev/hls.js/issues?q=is%3Aissue%20state%3Aopen%20label%3A%22Custom%20loader%22

TODOs:

• [ ] Add "Custom loader" label to older issues
• [ ] Identify implementation sticking points for all issues and address in docs
• [x] Response filter example (#7022)

1. Handle specific types of requests (playlists, fragments, other): 7fec594 a. config usage a. loader context usage
2. Providing loader stats (type structure and dependencies)
3. ~Extending existing loaders with composition vs inheritance~ (example covers inheritance and facade patterns)

Resolves issues:

Checklist

• [ ] changes have been done against master branch, and PR does not conflict
• [ ] new unit / functional tests have been added (whenever applicable)
• [ ] API or design changes are documented in API.md